Diphenylmethane dicarbamates and diphenylmethane bis(dialkylureas) are useful precursors to diphenylmethane diisocyanates. Either the dicarbamates (U.S. Pat. No. 3,962,302) or the bis(dialkylureas) (U.S. Pat. Nos. 3,936,484, 4,871,871, 4,873,364) decompose thermally and/or in the presence of a promoter to give the diisocyanates.
Conventional approaches to diphenylmethane dicarbamates focus on the condensation of an alkyl phenylcarbamate with formaldehyde or its equivalent in the presence of an acid catalyst. This approach is illustrated in U.S. Pat. Nos. 2,946,768, 4,162,362, 4,202,986, 4,287,132, 4,543,419, and in Japanese Kokai Nos. 2-83363 and 2-3658. Unfortunately, the reaction of alkyl phenylcarbamates with formaldehyde under acid conditions is complicated by side reactions. Formaldehyde reacts at the carbamate nitrogen to give undesirable N-(alkoxycarbonyl)phenylaminomethylphenyl compounds, or so-called "N-benzyl compounds," which include dimers, trimers, tetramers, etc. of these compounds. Polycondensation products--polymethylene polyphenylene dicarbamates--also form, and the formation of such polynuclear compounds is difficult to suppress. Consequently, it is difficult to selectively prepare diphenylmethane dicarbamates. Unlike TDI-like diisocyanates, diphenylmethane diisocyanates are not easily purified by distillation, so techniques such as fractional crystallization must be used to purify the diisocyanates or precursors. This fact makes good control over product selectivity especially critical. A preferred process is one that minimizes or eliminates polycondensation and other side products.
As an alternative to formaldehyde, alkylmethylene bis(alkyl carbamates) have been used as alkylating agents. Japanese Patent No. 63-203655 (1988) describes the preparation of polymethylene polyphenylene dicarbamates from the reaction of benzene and an alkylmethylene bis(alkyl carbamate). Murphy and Raman (J. Chem. Soc., Perkin Trans. I (1981) 447) show that ethyl phenylcarbamate reacts with methylene bis(ethyl carbamate) in the presence of boron trifluoride etherate to give a mixture of 4-(N-ethoxycarbonylaminoethyl)-N-ethoxycarbonylaniline (II) and diphenylmethane-4,4'-bis(ethyl carbamate) (III): ##STR1## Reaction in refluxing benzene over 24 hours gives 43% conversion of (I), with 77% selectivity to the intermediate product (II) and 23% selectivity to the MDI dimer precursor (III). Although the desired MDI dimer precursors are obtained, conversion and selectivity are lower than desirable for commercial use.
A process for making diphenylmethane diisocyanate precursors with high conversion, relatively short reaction time, and high selectivity to dimer products is needed. Preferably, the conversion of the intermediate products is complete, and formation of undesirable N-benzyl compounds or polymethylene polyphenylene diisocyanate precursors is minimized or avoided.